The need for ballistic and blast protection for vehicles and personnel are becoming more increasingly complex. Modern weapons and their improvised variants utilize high-amplitude, overpressure waves and high-explosive projectiles to cause damage to vehicles and people. High-explosive projectiles can propagate at much higher velocity and therefore carry more kinetic energy than bullets fired from a rifle, which are launched by propellants with a lower detonation velocity. Adversaries are able to quickly change the combination of blast and projectile loading of a structure and so it is necessary to develop armor solutions that can be quickly and inexpensively modified to meet these changing threat environments. Hence, a significant interest in developing armor solutions that offers protection against fragment and air blasts are needed.
These solutions must also be able to defeat more conventional projectiles such as bullets (including armor piercing and higher caliber, heavy machine gun rounds). If these solutions are to be utilized on mobile platforms it is essential that they mitigate a specific threat level at the lowest possible mass per unit area of protection (i.e., at the lowest specific mass or aerial density of the armor. Periodic Cellular Materials (PCM) materials are an emerging class materials and structures that are being studied for light weight structures and other multifunctional applications such as thermal management.
Commercially available stochastic foams can be effective at shock mitigation but suffer from severe limitations for the most demanding structural and multifunctional applications because they have low strength (strut bending) modes of failure. As a result, polymer and metal foams exhibit very limited crush resistance during static or dynamic compression.
Honeycombs can also be problematic for many applications because the stress needed to initiate core crushing during shock mitigation is much higher than the stress required to cause cell collapse. This results in larger force transmission through the structure. They also have a closed cell topology which can make them susceptible to corrosion and delamination.
In the low core weight configurations of most interest, the webs fail by elastic buckling which makes inefficient use of the core material in a sandwich panel construction intended for load support.